Integrated wood preserving processes for artificially drying wood products and subsequently impregnating the dried wood products with a preservative agent under pressure are old and well-known. One such known integrated process utilizes a boiling under a vacuum step to dry the wood products and a pressure step to thereafter force preservative agent into the cells of the dried wood products. This process has particular applicability to the drying and impregnation of wood products such as bridge ties, mine ties, railroad crossties and switch ties, pilings for both land and water use, utility poles, and the like wherein naphthalene containing creosote, creosote-coal tar and creosote-petroleum oil solutions are utilized in both the drying and the pressure steps.
In utilizing the above integrated process, the wood products to be treated first are loaded into a suitable treatment vessel known in the wood preserving art as a cylinder or retort. A preservative agent, such as one of the aforementioned naphthalene containing solutions, then is added to the treatment vessel to completely cover the wood products. Heat and vacuum then are applied to the treatment vessel to commence the drying of the wood products. Under these operating conditions water comprising the moisture inherent in the wood products continuously is boiled out of the wood products and withdrawn from the treatment vessel in the form of steam. This heat and vacuum treatment is continued until such time as a predetermined amount of water has been recovered from the treatment vessel.
Upon completion of this drying step the dried wood products then are subjected to a pressure impregnation step. For this impregnation step, the preservative agent either will be retained in the treatment vessel and a pressure applied thereto or the vessel first will be drained, pressurized to atmospheric or superatmospheric pressure, and the vessel then refilled with preservative agent. The particular impregnation procedure employed will depend upon whether the goal is to retain as much of the preservative agent in the cells of the wood products as is possible, or whether the goal merely is to coat the interior walls of the cells of said wood products. In the wood preserving art, these different impregnation procedures generally are referred to as the "full cell" process as exemplified by the known Bethel process and the "empty cell" process as exemplified by the known Lowry and Rueping processes.
Regardless of which of the above particular impregnation procedures is employed, elevated pressures are used in the treatment vessel to force the preservative agent into the wood products undergoing treatment. During this pressure step, heat also is applied to the treatment vessel to maintain the temperature of the preservative agent therein at a temperature level of approximately the same as that previously employed in the drying step. On completion of this pressure step, the treatment vessel then is drained of the preservative agent and most usually a vacuum applied to dry the surface of the wood products prior to their removal from the cylinder.
In utilizing the above integrated process for drying and impregnating wood products with one of the aforementioned naphthalene containing preservative agents problems can and do occur. These include, for example, the plugging of process equipment and the contamination of process derived waste waters with naphthalene as a result of the naphthalene being stripped from the preservative agent by the moisture removed from the wood products during the drying step. These problems further are compounded by the fact that in most commercial operations, it is usual practice to recover this naphthalene and return it to the preservative agent storage or supply tanks. Thus, with each succeeding use of the preservative agent from the storage tanks, the sample naphthalene not only is being restripped from the preservative agent during each drying phase, but also this napthalene is building up in concentration in the recycling preservative agent. Such build-up only further contributes to the aforementioned process equipment plugging and waste water contamination problems.